Back-lighted display devices have become popular for a variety of display purposes. Some of these displays are large and some small. The actual light for such devices is provided, as the name implies, from behind (the side away from a viewer). Light emitting diodes (LEDs) are now increasingly being used to provide the light source in such back-lighted applications.
An example of an LED lighted application is a liquid crystal display (LCD) where a number of LEDs are spatially distributed behind the LCD so that their light transmits through the LCD. The light from each LED is filtered by the LCD to provide red, green and blue pixels.
LED devices are directional light sources where the light is typically directed towards the central axial direction of the LED. A common problem in using LEDs as back-lights is that the resultant LCD display does not display uniform luminance, i.e., the display surface brightness is not uniform. The region directly above the LED device typically is brighter than the rest. These “hot spots” do not have a pleasant look.
Another common problem is that the light coming from the LED devices are poorly mixed. In a back-lighted display, red, green and blue LED devices are typically used and the different colors must be uniformly mixed to ensure a uniform color, for example, white color, is obtained. If color mixing from the LCDs is poor, then different portions of the LCD show different colors.
One prior solution to prevent hot spots is to place a diffusing film above the back-lighted array. This solution helps minimize, but does not eliminate the problem. The diffusing film attenuates the amount of light transmitted through the LCD and thus the overall brightness of the back-light module drops.
A prior solution to ensure more uniform color mixing is to increase the distance between the LED devices and the LCD display so that a larger light mixing zone is created. This increases the chance of light mixing before reaching the LCD at the expense of increased device thickness.